1. Field of the Invention
The present invention relates to a communication apparatus having a function for generating a call signal or a communication apparatus to which another terminal can be connected.
1. Related Background Art
An INT or IR signal (repetition of ON for one second and OFF for two seconds) for interrupting a voltage of 75 V (16 Hz) is used as a call signal for ringing a bell of a telephone set. FIG. 3 is a circuit diagram showing a principle for ringing a bell of a telephone set.
FIG. 4 is a diagram showing a conventional signal generating circuit for ringing a bell of a telephone set.
This conventional signal generating circuit comprises a 16-Hz oscillator 18, an amplifier 19, a control circuit 21, a gate 22 enabled/disabled in response to a control signal from the control circuit 21, an output transformer 20 for boosting an output from the amplifier 19, and a ring trip detect circuit 16 for monitoring whether output terminals 15-T and 15-R connected to a telephone set are engaged by a DC circuit. The ring trip detect circuit 16 is a circuit for detecting a DC loop formed during transmission of a 16-Hz call signal (one second). The detect circuit 16 integrates signals at the output terminals 15-T and 15-R and detects formation of a DC loop by determining a level of an integrated signal.
In the conventional signal generating circuit, in order to ring a bell of a telephone set connected to the output terminals 15T and 15R, a control signal shown in FIG. 5(h) is supplied from the control circuit 21 to the gate 22. An output signal from the gate 22 is shown in FIG. 5(j). This signal is amplified by the amplifier 19, thereby obtaining an amplified signal shown in FIG. 5(k). The amplified signal is supplied to the boosting transformer 20 and the output terminals 15-T and 15-R, thereby ringing the bell of the telephone set.
When a called party pickes up a handset of the telephone set, a DC circuit is formed. This DC circuit is detected by the ring trip detect circuit 16. A detection signal from the ring trip detect circuit 16 is supplied to the control circuit 21. The control circuit 21 stops generating the control signal supplied to the gate 22, and therefore the bell stops ringing.
In the conventional arrangement, since the boosting transformer 20 is used at a low frequency of 16 Hz, utilization efficiency of the power source is poor. In addition, in order to reduce a loss of the boosting transformer 20, the boosting transformer becomes bulky. As a result, the boosting transformer is expensive, and the mounting volume of the call signal generating circuit is undesirably increased.
Telephone lines have been used for data communication of a facsimile machine and a teletex in addition to speech communication. A switching relay is arranged in a conventional facsimile machine to selectively connect a telephone set and the facsimile machine to the telephone line. The switching relay is switched under the control of a controller. In an automatic reception mode, when a call signal is received from a telephone line, the line is automatically switched from the telephone set to the facsimile machine to receive an image signal.
The present applicant proposed a communication system for connecting the above-mentioned facsimile machine to a telephone set for exclusive use in a key telephone system in U.S. patent application Ser. No. 270,208. In this USSN, a call signal generating circuit and a ring trip detect circuit shown in FIG. 4 are arranged in the telephone set for exclusive use to automatically start the facsimile machine. However in this case, high-breakdown electronic components must be used in the call signal generating circuit and the ring trip detect circuit, as described above.
In addition, generation of a dummy call signal and dummy response detection of the facsimile machine by ring trip detection must be performed. As a result, control procedures are undesirably complicated.
In U.S. Ser. No. 270,208, the following procedures must be performed when a facsimile communication destination is called from a telephone set for exclusive use by using its dialing function. A basic arrangement of the system shown in U.S. Ser. No. 270,208 is shown in FIG. 6.
(1) A handset of a telephone set 63 for exclusive use is set in an off-hook state to form a DC loop of a speech communication line 66 and an external line 61.
(2) A telephone number of a destination to be called (speech communication destination) is input with telephone number buttons of the telephone set for exclusive use and is sent out onto the external line 61.
(3) When a response of the destination facsimile machine is confirmed, the telephone set for exclusive use is operated to form a speech communication path by connecting the speech communication line 66 and a speech communication line 68 upon formation of a DC loop of the speech communication line 68 for connecting the telephone set for exclusive us and the facsimile machine.
(4) When the handset of a telephone set 65 for general use is set in an off-hook state and a start button of a facsimile machine 64 is depressed, a DC loop is formed in the speech communication line 68. Therefore, the speech communication line 66 is connected to the speech communication line 68 to set a facsimile communicable state.
(5) Facsimile communication is performed between the facsimile machines.
The above system, however, poses the following problems to be solved.
(1) Both the telephone set 63 for exclusive use and the telephone set 65 for general use must be prepared.
(2) Unless the handset of the telephone set for general use is set in the off-hook state upon switching with the telephone set 63 for exclusive use, the start button of the facsimile machine 64 cannot be operated, thus inevitably requiring complicated procedures.